说明

Add a signed amount to a field. Adding a positive amount allows advances in
time, even if the numeric value of the field decreases (e.g. when working
with years in BC dates).

Other fields may need to adjusted – for instance, adding a month to the 31st
of January will result in the 28th (or 29th) of February. Contrary to
IntlCalendar::roll(), when a value wraps around, more
significant fields may change. For instance, adding a day to the 31st of
January will result in the 1st of February, not the 1st of January.

参数

The signed amount to add to the current field. If the amount is positive,
the instant will be moved forward; if it is negative, the instant will be
moved into the past. The unit is implicit to the field type. For instance,
hours for IntlCalendar::FIELD_HOUR_OF_DAY.